Geochronology Project: Zircon coronas in Proterozoic massif-type anorthosite!
Proterozoic massif-type anorthosite, renowned for its restricted time occurrence, has long been suggested that can provide a unique perspective on the global crust-mantle interactions and evolution through time (Ashwal and Bybee, 2017). However, the source of their parental magma, whether from the mantle or lower crust, remains the subject of ongoing debates. Further, in light of the recent new evidence of ~120 Ma gap between different mineral assemblages (Bybee et al., 2014, 2019), critical debates have been raised concerning the length of its magmatic process (e.g., Vander Auwera et al., 2014; Ashwal and Bybee, 2017).
Despite their wide occurrences in all continents, only one such complex (Damiao anorthosite) has been identified in China. In this new contribution, we characterize the mineralogical features and petrogenesis of the second Proterozoic anorthosite in China: Daxigou anorthosite complex (DAC) in northern Tarim craton. In particular, we performed the first in situ petrographic thin section U–Pb dating of zircon coronas in Proterozoic anorthosite, the age of which is used to constrain the closure time of the DAC. We then explore the starting time of the DAC by dating the diabase after showing it is the possible parental magma. We found these two ages are indistinguishable within uncertainties, thus favoring a rapid construction of the Proterozoic anorthosite. The trace elements and Sr–Nd–Hf isotopes indicate the DAC was formed by a small degree partial melting of the enriched sub-continental lithospheric mantle induced by a post-collisional slab break-off. The documented new anorthosite, along with other contemporaneous mafic dykes and rapakivi-type granites in south Tarim craton, supports the connection of the North and South Tarim orogen during the assembly of the Columbia supercontinent.
Despite their wide occurrences in all continents, only one such complex (Damiao anorthosite) has been identified in China. In this new contribution, we characterize the mineralogical features and petrogenesis of the second Proterozoic anorthosite in China: Daxigou anorthosite complex (DAC) in northern Tarim craton. In particular, we performed the first in situ petrographic thin section U–Pb dating of zircon coronas in Proterozoic anorthosite, the age of which is used to constrain the closure time of the DAC. We then explore the starting time of the DAC by dating the diabase after showing it is the possible parental magma. We found these two ages are indistinguishable within uncertainties, thus favoring a rapid construction of the Proterozoic anorthosite. The trace elements and Sr–Nd–Hf isotopes indicate the DAC was formed by a small degree partial melting of the enriched sub-continental lithospheric mantle induced by a post-collisional slab break-off. The documented new anorthosite, along with other contemporaneous mafic dykes and rapakivi-type granites in south Tarim craton, supports the connection of the North and South Tarim orogen during the assembly of the Columbia supercontinent.